Sen. Barack Obama, groping for a position from which to differentiate his position on the war in Iraq from Hillary’s, has turned to the past to find traction.

“There is a choice that has emerged in this campaign, one that the American people need to understand,” he said recently at DePaul University. “They should ask themselves: Who got the single most important foreign-policy decision since the end of the Cold War right, and who got it wrong?”

This question is fine for a history test, but it lacks relevance to the current presidential campaign, where both Hillary and Obama and, for that matter, John Edwards, are saying similar things about the war in Iraq.

But Obama has an opening for a very effective challenge to Hillary over the war if he uses the half-hour interview she gave The New York Times in her Senate office on March 14th of this year. In that on-the-record session, Hillary let slip her real colors on Iraq, revealing her essentially hawkish approach to the issue.

She said that she foresees a “remaining military, as well as political, mission” in Iraq. She said she “would keep a reduced but significant military force there to fight al Qaeda, deter Iranian aggression, protect the Kurds and possibly support the Iraqi military.”

While noting that in public Sen. Clinton backs the goal of “bringing the troops home,” the newspaper said that she took a “more nuanced position” in the interview and spoke of the “remaining vital national security interests in Iraq” that would require an ongoing deployment of troops.

That interview, a major mistake by Clinton, provides enough of an opening to drive a truck through and gives Obama a perfect way to position himself vis-à-vis her. The Illinois senator should quote his opponent and ask Clinton to explain to the American people how many troops she would keep in Iraq and for how long.

Hillary would, of course, parry the attack by declining, as she always does, “to discuss hypotheticals,” but if Obama pounds away at the point, her disinclination to address specifics will seem evasive to the antiwar left of the Democratic Party. It would become evident as the exchange between Hillary and Obama on the issue unfolds that she would not pull the troops out but would leave very significant numbers there for a very long time.

It is a short leap to imagine that a President Hillary Clinton would find it necessary to reinforce the American garrison in Iraq to make it less vulnerable to attack. Indeed, the extensive missions she enumerated in her interview leave open the possibility of a long continuation of the war.

It is on this issue – in the present and future tense – that Obama should harp, rather than trying to dig up the past to find differences between him and the front-runner.

How many troops would it take to deter Iranian aggression? What if Iran stepped up its attacks? What support would you offer the Iraqi army? Logistical? Intelligence? Training? Transportation? Air cover? And to what lengths would you go to protect the Kurds? What if Turkey crosses the border and attacks them? And how many troops would you allocate to fighting al Qaeda? Do we read your commitment to this mission as indicating that you would not reduce troop levels in Anbar Province where the battle with Al Qaeda is being waged?

These are the questions Obama must pose. By raising and repeating them, the media will pick them up and Hillary will be cast on the defensive on the most important issue in the Democratic Primary.

Of course, Obama has been as opaque as Hillary about his plans for Iraq. Like her, he agrees that we need to keep an ongoing presence in the country, and, like her, he won’t specify what he wants to do there and how many troops it would take.

But Obama does not have the luxury of ambiguity. His campaign is going no place fast. He is failing to make inroads into Hillary’s support, and his focus on the past, as opposed to the future, is a pathetic attempt to find an issue by archeology. He should decide what he wants to do in Iraq and challenge her to be specific about her plans. Her New York Times interview traps her into a more extensive involvement than Obama will likely seek. It now remains for him to close the trap.

This is a strategy that Obama can use to dent Hillary’s lead and move up in the polls.